A steam temperature is less than 600° C. in a steam device in which high-temperature steam is passed, such as a steam turbine of a conventional thermal power generating plant. Therefore, a ferritic heat-resistant steel is generally used considering economical efficiency and manufacturability for the major parts of high temperature portions (such as turbine rotors, moving blades, etc.) of the steam turbine.
To provide the thermal power generating plant with high efficiency in view of the environmental conservation in these years, steam turbines using high-temperature steam of about 600° C. are being operated. In such steam turbines, the steam temperature is increased to a high level, so that the high-temperature strength of the ferritic heat-resistant steel is insufficient. Therefore, a heat-resistant alloy mainly made of nickel or an austenitic heat-resistant steel is used for some of the steam turbines.
At present, a steam turbine using higher-temperature steam of 650° C. or more is also being considered. In view of economical efficiency and manufacturability, there are disclosed technologies that a steam turbine power plant is configured with portions using heat-resistant alloys and austenitic heat-resistant steels decreased as much as possible.
The steam turbine power plant has a superhigh-pressure turbine portion, a high-pressure turbine portion, an intermediate-pressure turbine portion, a low-pressure turbine (1), a low-pressure turbine (2) and a generator connected to a single axis, and the superhigh-pressure turbine and the high-pressure turbine are independently built into the same outer casing. In this steam turbine power plant, use of the heat-resistant alloy and the austenitic heat-resistant steel is limited to a particularly high temperature portion of the superhigh-pressure turbine portion.
But, to realize a high temperature such that a steam temperature exceeds 700° C., only an increase of the heat-resistant temperature of the base material metal is limited, and a technology to cool high-temperature parts by the cooling steam is indispensable. There is a disclosed patent related to the above cooling technology.
In the field of gas turbines, there has been used a thermal barrier coating technology to cool the inner surfaces of high temperature parts by forming a low heat conductive ceramics layer on the surfaces in order to protect members using a Ni-based superalloy or a Co-based superalloy having high strength from a high temperature combustion gas. It is general to use a thermal spraying method to form the ceramics layer, but it is also considered to use a slurry/gel coating method using a ceramics precursor in order to smoothen the surface. But, since steam is heat-emitting gas due to radiation of infrared rays in the steam turbine, there are technically different problems that radiant heat transmission becomes more significant, not only a heat receiving member but also a heat radiating member are required to have thermal barrier performance. And, a ceramics thermal barrier coating for the gas turbine according to the mainstream thermal spraying method has pores in the ceramics layer to realize low heat transmission. But, it is worried that the steam turbine has a problem that a thermal conductivity increases because steam having a high thermal conductivity enters into the pore portion.
For the steam turbine having a steam temperature of exceeding 700° C. described above, various methods have been considered to assure the strength of turbine component parts. In conventional thermal power generating plants, the improved heat-resistant steel is being used for turbine component parts such as a turbine rotor, a nozzle, a moving blade, a nozzle box (steam chamber), a steam inlet pipe and the like used for the steam turbine. But, if the steam temperature exceeds 700° C., it is hard to assure the strength of the turbine component parts by the heat-resistant steel.
Therefore, for the steam turbine, it is expected to have a technology that a conventional improved heat-resistant steel having excellent economical efficiency and reliability is used for the low-temperature portions, a material having high heat resistance is limited to be used for the portions exposed to the high-temperature steam, and cooling steam is introduced between them. But, for example, to cool down the turbine rotor and the casing by the cooling steam in order to apply the conventional material to the member corresponding to a first stage of the turbine, a cooling steam amount corresponding to several percents of the main stream of steam is necessary. And, a flow of the cooling steam into the steam passage portion has a problem of lowering the internal efficiency of a single turbine involved in degradation of total performance.